Não existe praticamente possibilidade disso.
500 km é um planeta. Não há nenhum corpo no sistema solar com esse diâmetro que cruze com a Terra. Só se viesse de fora do sistema solar (aparentemente raro) e nesse caso seria ainda mais raro colidir logo com a Terra (em vez de Júpiter por exemplo).
Agora asteróides de 1km chocam já frequentemente com a Terra. Há 250 milhões de anos consta-se já 1 ou 2 possíveis impactos de corpos de aprox. 40km, o que varre 95% da vida terrestre. Há 65 milhões consta-se o famoso asteróide dos dinossauros que teria pelo menos 10 a 15 km, mas prov. de material metálico e denso, logo com muito mais efeito.
Consta-se que houve um impacto em 10900 BC que desencadeou uma mini glaciação e provavelmente contribuíu para a extinção parcial dos mamutes e de humanos também.
E à volta de 3000 BC há pelo menos duas colisões (3123 BC e 2807 BC) de corpos de cerca de 500 metros a 1 km, que causaram extinções massivas das civilizações e provavelmente a lenda da inundação global, dado que um caíu no oceano Indico e provavelmente causou tsunamis de cerca de 60 metros de altura (quase 6-8 x mais que o da Indonésia).
Curiosamente foi a partir destas datas que muitas civilizações colapsaram e muitas outras começaram de novo.
Segundo investigações que fiz na net ainda parece ter ocorrido 1 ou 2 impactos à volta de 2200 BC, mas mais pequenos (~100m diâmetro) e ambos para os lados do Médio Oriente. Estes já não causam grande impacto global embora possam talvez reduzir as temperaturas nos anos a seguir à colisão.
Já em tempos mais recentes são frequentes os impactos de umas dezenas de metros (10-100 metros), como o de Tunguska em 1908 e há outros que causaram crateras como por exemplo na Alemanha em 200 BC. Ver http://www.spacedaily.com/news/comet-04l.html
Com este tamanho os corpos são mais vulgares mas provavelmente explodem na atmosfera como aconteceu em Tunguska.
De acordo com a pura estatística, não é de estranhar que surja mais uma destas colisões mundiais nos próximos milénios. Mudaria a civilização muito drasticamente mas a humanidade continuaria.
A 10 meter-wide asteroid named 2009 BD discovered earlier this month is making a slow pass of the Earth, coming within 400,000 miles (644,000 km) of our planet. The near-Earth asteroid (NEO) poses no threat to us, but it is an oddity worth studying. Astronomers believe the rock is a rare "co-orbital asteroid" which follows the orbit of the Earth, not receding more than 0.1 AU (15 million km) away. It is stalking us.
On looking at the NASA JPL Small-Body Database orbital plot, it is hard to distinguish between the orbital path of the Earth and 2009 BD, showing just how close the asteroid is shadowing the Earth on its journey around the Sun…
In 2006, NASA announced that Earth's "second moon" was an asteroid called 2003 YN107 (with a diameter of about 20 meters) and it was about to leave the vicinity of Earth, leaving its "corkscrewing" orbit around our planet for seven years, only to return again in 60 years time. 2003 YN107 was of no threat (and wont be in the future), but it is interesting to study these bodies to understand how they interact with Earth. Having NEOs in stable orbits around the Earth could be of benefit to mankind in the future as missions can be planned, possibly sending mining missions to these rocky visitors so we can tap their resources.
The orbital path of 2009 BD (blue line) (JPL Small Body Database)
So far, little is known about the new 10 meter asteroid in our near-Earth neighbourhood, but it provides us with an exciting opportunity to track its laborious orbit to see whether it will eventually be ejected after making a close pass to the Earth's gravitational field (as was the case with 2003 YN107 in 2006). From preliminary observations, 2009 BD is projected to shadow our planet for many months (possibly years) to come. Until November 2010 at least, the asteroid will hang around the Earth, within a distance of 0.1 AU.
It is worth emphasising that 2009 BD is of no threat to the Earth, its closest approach takes it 644,000 km from us. For comparison, the Moon's apogee is 400,000 km, so 2009 BD is stalking us from afar, beyond lunar orbit.
As time goes on, astronomers will be able to track 2009 BD's orbit with more precision (for updates, keep an eye on the JPL Small-Body Database), but for now, we have a micro-second moon following the Earth on its orbit around the Sun…
Comet Lulin (C/2007 N3) is approaching Earth for a 38 million mile close encounter in late February. At that time, the comet could brighten to naked eye visibility (5th magnitude). Meanwhile, it's a nice target for backyard telescopes:
Chris Brennan of Barbados took the picture using a 7-inch telescope on Jan. 24th. "Note the double tail," he says.
Martin McKenna sends this report from Maghera, Northern Ireland: "I checked out Comet Lulin this morning before dawn using a 8.5-inch reflector. My immediate impression was that the comet is very bright; the coma is healthy and active with an obvious green color. [Note: The 'coma' is the comet's gaseous atmosphere.] Seeing both tails at the same time was quite a treat! I also looked at the comet using a pair of binoculars and despite the poor quality of the opticsI was still able to find the comet easily. Finally, I tried very carefully to detect it with the naked eye but I just couldn't convince myself that it was visible. However, I suspect that with excellent sky conditions the first naked eye observations will be reported very soon. This comet could very well put on a good show in February!"
Site UniverseToday disse:Today, January 14, the comet is at perihelion, closest to the sun. As it moves to its closest approach to Earth on February 24, Lulin is expected to brighten to naked-eye visibility in rural areas, (at best about magnitude 5 or 6) and will be observable low in the sky in an east-southeast direction before dawn.
Comet Lulin is now visible to the naked eye from dark-sky sites. "This morning, I noticed a faint smudge above Zubenelgenubi," reports Jeff Barton from the Comanche Springs Astronomy Campus in West Texas. "I then trained my 9x63 binoculars on the fuzzy patch. Yep, nailed it! I was thrilled to finally bag Comet Lulin without optical aid."
If optical aid is offered, however, don't refuse it. The view through a small telescope is dynamite:
"Wow, it was nice!" says photographer Mike Broussard of Maurice, Louisiana, who took this picture of Comet Lulin gliding by double star Zubenelgenubi on Feb. 6th.
Another report of naked-eye visibility comes from Martin McKenna of Maghera, Northern Ireland: "I went out for a look at Comet Lulin this morning before dawn with my telescope and binoculars. The Moon was very low, so I stood within the shadow of my house and tried to see the comet without optical aid. Using averted vision, I was able to glimpse the comet perhaps a dozen times! It looked like a large grey patch of light very close to Zubenelgenubi. The sight gave me a warm glow on such a frigid frosty night."
Comet Lulin rises in the east just a few hours before the sun. See for yourself: sky map.
Green Comet Approaches Earth
In 1996, a 7-year-old boy in China bent over the eyepiece of a small telescope and saw something that would change his life--a comet of flamboyant beauty, bright and puffy with an active tail. At first he thought he himself had discovered it, but no, he learned, two men named "Hale" and "Bopp" had beat him to it. Mastering his disappointment, young Quanzhi Ye resolved to find his own comet one day.
And one day, he did.
Fast forward to a summer afternoon in July 2007. Ye, now 19 years old and a student of meteorology at China's Sun Yat-sen University, bent over his desk to stare at a black-and-white star field. The photo was taken nights before by Taiwanese astronomer Chi Sheng Lin on "sky patrol" at the Lulin Observatory. Ye's finger moved from point to point--and stopped. One of the stars was not a star, it was a comet, and this time Ye saw it first.
Comet Lulin, named after the observatory in Taiwan where the discovery-photo was taken, is now approaching Earth. "It is a green beauty that could become visible to the naked eye any day now," says Ye.
Amateur astronomer Jack Newton sends this photo from his backyard observatory in Arizona:
"My retired eyes still cannot see the brightening comet," says Newton, "but my 14-inch telescope picked it up quite nicely on Feb. 1st."
The comet makes its closest approach to Earth (0.41 AU) on Feb. 24, 2009. Current estimates peg the maximum brightness at 4th or 5th magnitude, which means dark country skies would be required to see it. No one can say for sure, however, because this appears to be Lulin's first visit to the inner solar system and its first exposure to intense sunlight. Surprises are possible.
Lulin's green color comes from the gases that make up its Jupiter-sized atmosphere. Jets spewing from the comet's nucleus contain cyanogen (CN: a poisonous gas found in many comets) and diatomic carbon (C2). Both substances glow green when illuminated by sunlight in the near-vacuum of space.
In 1910, many people panicked when astronomers revealed Earth would pass through the cyanogen-rich tail of Comet Halley. False alarm: The wispy tail of the comet couldn't penetrate Earth's dense atmosphere; even it if had penetrated, there wasn't enough cyanogen to cause real trouble. Comet Lulin will cause even less trouble than Halley did. At closest approach in late February, Lulin will stop 38 million miles short of Earth, utterly harmless.
To see Comet Lulin with your own eyes, set your alarm for 3 am. The comet rises a few hours before the sun and may be found about 1/3rd of the way up the southern sky before dawn. Here are some dates when it is especially easy to find:
Feb. 6th: Comet Lulin glides by Zubenelgenubi, a double star at the fulcrum of Libra's scales. Zubenelgenubi is not only fun to say (zuBEN-el-JA-newbee), but also a handy guide. You can see Zubenelgenubi with your unaided eye (it is about as bright as stars in the Big Dipper); binoculars pointed at the binary star reveal Comet Lulin in beautiful proximity. [sky map]
Feb. 16th: Comet Lulin passes Spica in the constellation Virgo. Spica is a star of first magnitude and a guidepost even city astronomers cannot miss. A finderscope pointed at Spica will capture Comet Lulin in the field of view, centering the optics within a nudge of both objects. [sky map]
Feb. 24th: Closest approach! On this special morning, Lulin will lie just a few degrees from Saturn in the constellation Leo. Saturn is obvious to the unaided eye, and Lulin could be as well. If this doesn't draw you out of bed, nothing will. [sky map]
Ye notes that Comet Lulin is remarkable not only for its rare beauty, but also for its rare manner of discovery. "This is a 'comet of collaboration' between Taiwanese and Chinese astronomers," he says. "The discovery could not have been made without a contribution from both sides of the Strait that separates our countries. Chi Sheng Lin and other members of the Lulin Observatory staff enabled me to get the images I wanted, while I analyzed the data and found the comet."
Somewhere this month, Ye imagines, another youngster will bend over an eyepiece, see Comet Lulin, and feel the same thrill he did gazing at Comet Hale-Bopp in 1996. And who knows where that might lead...?
"I hope that my experience might inspire other young people to pursue the same starry dreams as myself," says Ye.
Space Rock 2009 DD45 Buzzes Earth
Late word out of the IAU's Minor Planet Center: a small asteroid will pass close to Earth tomorrow (March 2nd) at 13:44 Universal Time. How close? The MPC's Timothy Spahr calculates that it'll be 0.00047 astronomical unit from Earth's center. That's only about 40,000 miles (63,500 km) up — well inside the Moon's orbit and roughly twice the altitude of most communications satellites!
This little cosmic surprise, designated 2009 DD45, turned up two days ago as a 19th-magnitude blip in images taken by Rob McNaught at Siding Spring Observatory in Australia. It was already within 1½ million miles of Earth and closing fast.
Thankfully, the news media have become less sensationalistic when it comes to these asteroidal close calls — especially since one actually struck our planet last October 7th, at night, and the impact went virtually unnoticed.
So why post this? Well, we figured someone might want to watch it zip by at up to a half degree per minute! Even though it's small, likely no more than 100 feet (30 meters) across, it'll brighten to magnitude 10½ at its closest — easily within reach of an 8-inch backyard telescope. That's the good news.
The bad news is that the point of closest approach occurs over the Pacific somewhere west of Tahiti, so the most likely viewers are in Australia, Japan, and maybe Hawaii. Sure, you could look before or afterward, but it'll be brighter than 13th magnitude only for a few hours.
If you want to give it a try anyway, you'll need to generate an ephemeris of positions for your specific location, due to the wide parallax of an object so close to Earth. And if you manage to spot it, be sure to add a comment below to let us know.
By the way, this isn't the closest "near-miss" asteroidal fragment on record. According to the MPC, tiny 2004 FU162 skirted just 4,000 miles from us on March 31, 2004.
ASTEROID FLYBY: Newly-discovered asteroid 2009 DD45 is about to fly past Earth only 72,000 km (0.000482 AU) away. That's about twice the height of a typical geostationary communications satellite. The 30- to 40-meter wide space rock is similar in size to the Tunguska impactor of 1908, but this time there is no danger of a collision. At closest approach on March 2nd, around 1340 UT (5:40 am PST), 2009 DD45 will speed through the constellation Virgo shining as brightly as an 11th magnitude star. Experienced amateur astronomers can track the asteroid using this ephemeris.
UPDATE: Using a 14-inch telescope at the University of Nariño Observatory in Colombia, Alberto Quijano Vodniza has photographed the asteroid streaking toward Earth thirteen hours before closest approach: 1 MB movie.
This was taken by Joseph Brimacombe, Southern Galactic Telescope Hosting, on February 22, 2009. The solid part of a comet is usually pretty tiny, just a few kilometers across. Bigger than a mountain, but a lot smaller than a moon. But a lot of that solid part is frozen stuff that turns into gas when the comet nears the Sun. It expands, forming a cloud around the solid nucleus. That cloud can be huge– 100,000 kilometers across or more, bigger than planets!
The gas escapes away from the comet, forming a tail. That tail can get kinks, twists, sheets, ribbons, all kinds of shapes as it moves off. In the video, you really get a sense of the majesty and beauty of this process. As the tail fans out, sometimes it looks like we see part of it on both sides of the comet, but that’s just a perspective effect. Imagine someone with long hair in the wind; their hair forms a comet-like shape behind them. Looking straight at them, face-on, you’d see hair on both sides of their head. That’s more or less what we see with Lulin. You can see the so-called anti-tail in lots of the images posted of Lulin. Try a Google image search and be amazed. This is some gorgeous comet.